a. Field of the Invention
This invention relates to liquid cooled semiconductor modules.
b. Related Art
Cooling of integrated circuit chips in large computers is often accomplished placing the chips in a number of liquid cooled modules. FIG. 1 shows a cross-sectional view of a prior art thermal conduction module 10 for providing cooling of the integrated circuit chips 12 contained therein. As is well known, the power consumed in the circuits within the chips 12 generates heat which must be removed. The chips 12 may have different power requirements and different ranges of operating temperatures. Thus, in order to obtain reliable operation of the integrated circuits, the cooling must be of such character as to maintain the temperature of each chip within its required operating range.
The chips 12 are mounted (by way of solder balls 28) on one side of a substrate 14, generally made of ceramic, which has pins 16 extending from the other side thereof. These connecting pins 16 provide for the plugging of the module into a board (not shown) which may carry auxiliary circuits, etc. A housing or cap 18 is attached to the substrate 14 by means of a flange 20 which extends from the periphery of the substrate 14 to the cap 18. The cap 18 is made of a good heat conductive material such as copper or aluminum. The cap has small cylindrical openings 22 located therein, which are arranged in a matrix directly adjacent to the exposed surface of each chip 12.
Each of the openings 22 receives a spring loaded pin piston 24. Each piston 24 has a square header 26 which contacts the chip surface. Heat generated by the chips 12 is carried away by the pistons 24 to the cap 18. A cold plate 30, attached to or designed as an integral part of the cap 18, transfers the heat to a stream of coolant (for example chilled water) which flows from an inlet 32 to an outlet 34. The coolant carries the heat away from the thermal conduction module assembly 10.
As the power consumption of integrated chips has increased, the amount of heat which needs to removed from the chips and the thermal conduction module has increased as well. Since the ability of the thermal conduction module to remove heat from the chips is dependant on the ability of the cold plate to remove heat from the pistons and the TCM cap, a great deal of design effort has been put into developing an efficient cold plate design.
FIG. 2 shows a top-cutaway view of a prior art cold plate 200 having a serpentine coolant flow path 202 between an inlet 203 and an outlet 204. A number of bolt holes 205 are provided for attaching the cold plate to the cap of a thermal conduction module. One limitation of the cold plate of FIG. 2 is that the serpentine coolant flow path is generally serial. Thus, a single stream of coolant carries away heat from the chips as it passes over the pistons on its way from the inlet 203 to the outlet 204. Given such a serial flow path, in order to properly cool chips having higher power dissipations the rate of water flow must substantially be increased.
Increasing the flow rate to accommodate chips with high power dissipations can prove to be a difficult and costly exercise. In order to accomplish an increase of flow rate by a factor of X, the pressure drop between the inlet and the outlet of the TCM must be increased by X squared. Increases in pressure can be costly in several respects. For example, increasing coolant pressure typically requires larger and more expensive compression/cooling units. Further, suitable fittings and coolant supply tubing (typically more expensive) must be provided to accommodate the higher pressures.
Another problem with serial flow paths is that as the length of the path increases the required pressure drop must also be increased in proportional relationship to maintain a given flow rate. Thus, if the substrate to be cooled becomes larger, or the number of rows of integrated circuits on a given sized substrate increases, the longer flow path will cause a proportional increase in the required pressure drop.
FIG. 3 shows an example of another prior art cold plate 300 that uses fins for improved heat transfer. Four fins 302 extend from two opposite walls of the cold plate to form a sinuous path for water flow. Each parallel segment of the path is relatively wide and is further divided into four parallel channels by a three by three parallel line of short fins 304. The gaps between the short fins increase the turbulence in the water flow. The parallel channels provide increased fin area with lower flow resistance. While the embodiment of FIG. 3 is an improvement over that of FIG. 2, the coolant is still forced to flow along an essentially serial path over the pins of the TCM. Further, as the number of fins is increased the pressure drop required to maintain sufficient cooling is increased as well.